The present invention relates to resistance-reducing conductive adhesives, and to apparatus and methods of attaching electronic components using resistance-reducing conductive adhesives.
Semiconductor chips (or die) may be mounted to circuit boards or other electronic components in several ways. FIG. 1 shows a die 10 mounted to a circuit board 20 in a xe2x80x9cflip chipxe2x80x9d or xe2x80x9cchip on boardxe2x80x9d (COB) assembly. In this assembly, the die 10 has a pair of bond pads 12 that are attached directly to a corresponding pair of contact pads 22 on the circuit board 20. The bond pads 12 and contact pads 22 are typically formed from aluminum, although other electrically conductive materials may be used. The bond pads 12 may be attached to the contact pads 22 by soldering or by some other suitable method.
Electrical signals from the circuit board 20 may then be transmitted to the internal circuitry (not shown) of the die 10 through the contact pads 22 to the bond pads 12, and vice versa. It is customary to provide an encapsulating layer (or xe2x80x9cglob topxe2x80x9d) 14 over the die 10 to hermetically seal the die 10, thus insulating and protecting the die 10 from humidity, oxidation, and other harmful elements.
It is known to use a layer of conductive material to attach the bond pads of a die to the contact pads of a circuit board, as disclosed in U.S. Pat. No. 5,789,278, and in commonly-owned co-pending patent application number 09/389,862, both incorporated herein by reference. For example, FIG. 2 is a partial cross-sectional view of a bumped die 40 attached to a circuit board 20 using an anisotropically conductive layer 50. In this assembly, solder bumps 42 are formed on the bond pads 12 of the die 40. The anisotropically conductive layer 50 is formed between the bond pads 12 and the contact pads 22 on the circuit board 20.
The anisotropically conductive layer 50 includes a plurality of conductive particles 52 distributed in a suspension material 54, providing electrically conductive pathways 56 through the suspension material 54 in one direction (e.g. the xe2x80x9czxe2x80x9d direction as shown in FIG. 3). The conductive pathways 56 may be formed, for example, by compressing the solder bumps 42 against the layer 50, causing the conductive particles 52 to contact each other to form columns of conductive particles. Electrical signals are then transmitted from the circuit board 20 to the die 40 through the conductive pathways 56, and vice versa. The layer 50 is electrically insulative all other directions, hence it is xe2x80x9canisotropicallyxe2x80x9d conductive.
Anisotropically conductive layers 50 may be formed in a number of ways, including as a film or as a viscous paste that is applied (e.g. stenciled, sprayed, flowed, etc.) to the circuit board 20 and the contact pads 22. The anisotropically conductive layers 50 may then be cured by, for example, subjecting the suspension material 54 to certain environmental conditions (e.g. temperature, pressure, etc.), exposing to suitable curing compounds, irradiating with ultraviolet or ultrasonic energy, or other means depending on the composition of the suspension material 54. The suspension material 54 may be composed of a variety of materials, including thermoset polymers, B-stage (or xe2x80x9cpre-pregxe2x80x9d) polymers, pre-B stage polymers, thermoplastic polymers, or any monomer, polymer, or other suitable material that is non-conductive and can support the conductive particles 52. Various suspension materials are taught, for example, in U.S. Pat. No. 5,221,417 to Basavanhally and in U.S. Pat. No. 4,737,112 to Jin et al. The conductive particles 52 are commonly formed from silver, nickel, or gold, however, a variety of electrically conductive particles may be used.
Isotropically conductive layers may also be used for attachment of electronic components. For example, FIG. 3 is a partial cross-sectional view of a die 10 having a pair of bond pads 12, each bond pad 12 being attached to corresponding contact pads 22 of a circuit board 20 by an isotropically conductive layer 60. Like the anisotropically conductive layer 50 described above, each isotropically conductive layer 60 includes a plurality of conductive particles 62 suspended in a suspension material 64. The isotropically conductive layer 60, however, is electrically conductive in all directions and therefore does not extend between adjacent bond pads 12 (or contact pads 22) to prevent shorting or erroneous signals. Electrical signals from the circuit board 20 are transmitted through the isotropically conductive layers 60 to the die 10, and vice versa. Both isotropic and anisotropic conductive materials are commercially-available from, for example, Ablestik of Rancho Dominguez, Calif. or A.I. Technology, Inc. of Trenton, N. J. or Sheldahl, Inc. of Northfield, Minn. or 3M of St. Paul, Minn.
Although successful results have been achieved using the above-referenced die packages, there is room for improvement. For example, each of the electrical connections between the bond pads 12 and the contact pads 22 are electrically resistive which may reduce signal strength, increase power consumption, and increase waste heat generation. These characteristics may undesirably degrade the performance of an electronic assembly.
The present invention is directed to resistance-reducing conductive adhesives, and to apparatus and methods of attaching electronic components using resistance-reducing conductive adhesives. In one aspect, a resistace-reducing conductive adhesive comprises a first quantity of conductive adhesive, and a second quantity of a chelating agent combined with the conductive adhesive. The chelating agent reacts with a metal, typically an oxidized form of the metal such as an oxide or metal ion of a metal-containing conductive lead (or other electronic component) to form a soluble metal-ligand complex. The chelating agent may also react with an oxide-free form of the metal on the conductive lead to passivate the metal by forming hydrogen bonds. The resistance of the resulting electrical connection is reduced in comparison with prior art methods of conductive adhesive coupling, providing improved signal strength, reduced power consumption, and decreased waste heat.
In various alternate aspects, the conductive adhesive may comprise an anisotropically conductive adhesive, an isotropically conductive adhesive, a conductive epoxy, or a hydrophilic adhesive. In other aspects, the chelating agent reacts with a lead comprising another conductive material, particularly a metal. Typically, the metal is a divalent or trivalent metal, including but not limited to, aluminum, copper, gold, nickel, platinum or silver. In a preferred aspect, the metal is aluminum. Alternately, the chelating agents may be any suitable agent that provides the desired reactive mechanisms, including, for example, oxalic acid, malonic acid, citric acid, and succinate succinic acid. In a further aspect, the second quantity of the chelating agent comprises a value within the range from approximately 0.1 percent by weight to approximately 20 percent by weight, inclusive.
In another aspect, an electronic assembly comprises a first component having a first conductive lead formed thereon, a second conductive lead, and a resistance-reducing conductive layer extending between the first conductive lead and the second conductive lead. The resistance-reducing conductive layer comprises a conductive adhesive having a plurality of conductive particles disposed within a suspension material, and a chelating agent approximately uniformly blended with a portion of the conductive adhesive, the chelating agent being chemically reactive with an at least partially oxidized metal ion or metal-oxide to form a soluble conductive metal-ligand complex. The portion of the conductive adhesive may include substantially the whole volume of the conductive adhesive or be a local volume locally disposed between the conductive leads. Alternately, the first component may comprise a die, a circuit board, or any other electronic component.
In yet another aspect, a method of electrically coupling a first conductive lead of an electronic component to a second conductive lead comprises combining a conductive adhesive with a chelating agent to form a resistance-reducing conductive adhesive, the chelating agent being chemically reactive with a metal component of the first or second conductive leads to form a soluble metal-ligand complex, forming a layer of the resistance-reducing conductive adhesive proximate the first and second conductive leads, and engaging the first and second conductive leads with the layer of resistance-reducing conductive adhesive.